Screen printing has been used for producing a patterned display element comprising a color pattern formed on a metallic or plastic plate. A multi-colored pattern can be formed by repeating the printing operation by using a plurality of screens.
However, since screen printing employs a printing ink containing an organic solvent, it requires not only equipment for solvent recovery, drying, and the like but also requires much time for drying the ink, thus being poorly suited for continuous production efficiency. In particular, formation of a multi-colored pattern involves a drying treatment for each printing operation and, therefore, a long production line would be necessary for continuous operation to establish an automatic production system. Hence, adoption of such a continuous system has been substantially difficult.
Moreover, the working environment is easily contaminated with gases from the organic solvent. From hygienic standpoint, it is desirable that processes using an organic solvent be condensed into the smallest step, so as to exclude use of an organic solvent from the site of production of display elements. The requisite number of facilities for solvent recovery, drying, etc. would be reduced accordingly.
Accordingly, the present inventors have made an attempt to substitute the organic solvent-containing ink a solvent-free photocurable ink. However, such a photocurable ink turned out to exhibit radical polymerization inhibition due to atmospheric oxygen, failing to exhibit sufficient physical and chemical characteristics as demanded for production of a formed pattern.
On the other hand, a printing medium comprising a flexible substrate having provided thereon an ultraviolet-curable resin layer and a process for forming a pattern using such a printing medium have been proposed as described in JP-A-59-84240 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). In the above-described pattern forming process, the printing medium is exposed to light through a photomask resulting in a sheet having a latent image. The sheet is adhered by pressing to an adherend to transfer the uncured area to the adherend, and the transferred uncured area is then cured by exposure to light.
However, this printing system involves poor control of the surface smoothness and the evenness of the visual color of patterns. That is, when the sheet having a latent image is stripped or peeled off from the adherend, the surface layer of the uncured resin accompanies the sheet due to cohesive failure, and the surface of the ultraviolet-curable resin layer having undergone cohesive failure becomes a surface of a formed pattern. As a result, the formed pattern has a rough surface and a scattered thickness, which both cause color unevenness when seen by the eye.